The present invention relates generally to byte destuffing circuits for time division multiplexed digital signals.
Although it is generally intended that digital equipment be synchronized to each other or to a common clock, allowances must be made for the interworking of equipment that operates with slightly different clocks using elastic store. In SONET (Synchronous Optical NETwork) in which the signal is in byte interleaved multiple frame structure, positive and negative byte stuffing techniques are employed for frequency justification.
In a typical byte destuffing circuit, a buffer memory is provided for storing received data. Line clock is recovered from the received data and a write address generator is driven by the line clock to store each byte of the received data. A local clock is generated by a voltage-controlled oscillator to drive a read address generator for reading each byte of stored data. The read and write addresses are compared with each other by a phase comparator to generate a VCO control signal to synchronize the local clock with the line clock. A destuffing control signal is derived from a stuff byte multiplexed with the incoming signal. The write address generator is responsive to the destuffing control signal for destuffing eight successive pulses of line clock to suspend the generation of a write address for a byte location if a positive byte stuffing is effected at the transmit end.
However, one serious problem is that the phase comparator develops a large voltage change when the line clock is destuffed and there is a corresponding large frequency fluctuation in the local clock, resulting in a substantial amount of jitter in the data output of the buffer memory.